Aqueous urethane coatings

ABSTRACT

A crosslinked and chain extended urethane polymer coating with good performance properties typical of existing solvent borne solution coatings results from cure at ambient temperatures (e.g. 70° F.). The aqueous system is a primary amine-terminated urethane oligomer in which no less than about 25% and no more than about 40% of the amine functionality is substituted by a salt of a volatile acid which aids dispersion of the oligomer. Minimal organic solvent is present, and as the acid evaporates the product lacquer dries.

This application is a continuation-in-part of application Ser. No. 131,894, filed Dec. 11, 1987, now U.S. Pat. No. 4,772,643.

This invention relates to the formation of urethane polymer compositions employed as protective coatings for substrates in service conditions where good film performance characteristics are required. More specifically the present invention is identified as an aqueous dispersion of a urethane oligomer terminated with amine functionality. Dispersion of the urethane oligomer is facilitated by converting a limited amount of the terminal amines to a cationic ammonium salt, for example one that is derived from acetic acid.

BACKGROUND OF THE INVENTION; PRIOR ART

Polyurethane coatings are known to have the advantages of high gloss, chemical resistance and abrasion resistance. When formulated properly they can display flexibility, impact resistance and toughness. Additionally, when aliphatic or cycloaliphatic multifunctional isocyanate monomers are utilized in the composition, they exhibit outstanding weatherability and lightfastness characteristics.

A similar urethane oligomer that has terminal primary amine functionality and has been dispersed in water with the aid of a neutralizing, volatile organic acid such as acetic acid was disclosed in the filing of patent application 131,894. While the products disclosed herein are not in the high performance category, relative to mositure cure and two component urethane systems, they are good compared to the conventional coatings, such as house paints and other seal coatings.

As will be explained below, only minimal quantities of volatile organic solvents are needed in this invention, to the degree that the volatile organic content (VOC) falls well below current EPA requirements.

THE OBJECTIVES OF THE PRESENT INVENTION

The primary objective of the present invention is to design and develop aqueous urethane systems that are substantially free of hazardous volatile organic solvents and free diisocyanate monomers. Expressly, these coating systems can provide physical, chemical and water resistance properties of room temperature cured films similar to those found in conventional one component solvent borne coatings systems.

Another object of the present invention is to provide a urethane coating that is an aqueous dispersion of an amine terminated urethane oligomer having less than about 40% but more than about 25% by weight of the amine functionality neutralized to salt form, the VOC of which is well below the level established by the EPA as environmentally unacceptable. Volatile organic solvents are normally stripped from the urethane oligomer (to less than 1%), but a coalescing type solvent, or cosolvent, can be added, if required.

Another object of the present invention is to devise a aqueous urethane coating system as above described in which the desirable urethane properties of good gloss and good abrasion resistance can be developed.

An additional object of the present invention is to limit the degree of neutralization of the amine terminated urethane oligomer so that the drying will proceed as fast as possible without interfering with the urethane characteristics of the coating mentioned in the preceding paragraph.

Another object of the present invention is that by having the primary amine end group, coatings can be made than can be selectively stripped. That is they can have good detergent scrub resistance, but can be removed with certain acid solutions.

PREFERRED EMBODIMENTS

Primary amines react with isocyanate functional polymers in crosslinking reactions, one primary amine reacting with two isocyanate groups, causing almost instant gellation. A molecule containing primary amine functionality can be attached to an isocyanate terminated polyurethane, however, if it has one other functional group bearing one active hydrogen (e.g., secondary amine, hydroxyl, thio compound, etc.). The primary amine functionality is first blocked by the condensation reaction of the primary amine and the carbonyl of a ketone, forming a ketimine.

Monoprimary and/or polyfunctional primary amines that contain one other functional group bearing one active hydrogen atom, such as those of the formula

    H--X--(R.sub.1 NH.sub.2).sub.m (R.sub.2).sub.n

are preferred, wherein m represents the integers 1 or 2, while concurrently, n represents the integers 1 or 0. R₁ represents the residue of a mono or polyfunctional primary amine after removal of the primary amino nitrogen, and the removal of the functional group bearing one active hydrogen atom. R₁ may be aliphatic, cycloaliphatic, heterocyclic, or aromatic and may be saturated or unsaturated. R₁ may be extensively branched, and can bear one or more additional primary amine functional groups.

R₂ may be aliphatic, cycloaliphatic, heterocyclic, or aromatic, and may be saturated or unsaturated. R₂ may be attached to X, giving X a valence of 3, or it may be attached to R₁, giving X a valence of 2. R₂ may be a simple side chain for X or R₁, or can connect X and R₁ to complete a cycloaliphatic structure, or it can be terminated with a primary amine functional group. When R₂ is terminated by a primary amine functional group it may be identical to R₁ or it may be dissimilar.

X represents a functional group bearing a single active hydrogen atom that is reactive with free isocyanate groups; nitrogen, oxygen or sulfur. The literature lists a number of additional functional groups with single active hydrogen attachment (e.g. carboxylic acids, halogen acids, etc.), that interact with isocyanates, but currently, these are not preferred due to instability and undesired side reactions or decomposition products. The functonal group X and its single active hydrogen atom is therefore either a secondary amine, a hydroxyl, or a thio group. The entire molecule represented by the above formula is either a monosecondary, monohydroxy, or monothio, mono or poly functional primary amine.

Examples of monosecondary, monoprimary amines in commerce are limited. Compounds of this type that have been found suitable for the purposes of this invention are, although not limited to; N-(aminoethyl) piperazine, and N-methyl-1,3-propanediamine. Examples of monosecondary, polyfunctional primary amines in commerce are likewise somewhat limited. Diethylene triamine, containing two primary amine groups and one secondary amine group, is presented as one well-known example of this type of compound.

Suitable examples of monohydroxy, monoprimary amines are, although not limited to; monoethanolamine, monoisopropanol amine, 3-amino-1-propanol, and the like. A suitable example of a monohydroxy, polyfunctional primary amine is, although not limited to; 1,3-diamino-2-hydroxypropane.

Monothio, mono or polyfunctional primary amines can be prepared by the reaction of hydrogen sulfide or certain mercaptans with unsaturated monoamines; allyl amine, butenyl amine, cyclohexenyl amine, and the like. Examples of useful mercaptans for these syntheses include, among others, 1,3-propanedithiol, 1,4-butanedithiol, 1,4-benzenedithiol, and the like.

Before the mono or polyfunctional primary amines above can be used to terminate the free isocyanate functionality of an isocyanate functional prepolymer, however, the primary amine functional groups must first be blocked, or "masked" by reacting the primary amine functional groups with ketones, a condensation reaction wherein the carbonyl of the ketone combines with the two active hydrogens of the primary amino nitrogen forming water, which splits away, leaving an imine or polyimine such as those of the formula ##STR1## wherein ##STR2## is the residue remaining from a monoketone by the removal of the carbonyl, and wherein the two R radicals, when combined, contain from 2 to about 24 carbon atoms. The two R radicals preferably are aliphatic. They may be identical in structure, or may vary in size and/or configuration. Suitable examples of useful ketones include, among others, acetone, methylethyl ketone, diethyl ketone, methylisobutyl ketone, dibutyl ketone, diisobutyl ketone, methylisopropyl ketone, methyloctyl ketone, ethylbutyl ketone, dioctyl ketone, and the like. The ketimines or polyketimines may be prepared by methods disclosed in U.S. Pat. No. 3,291,775, and in Example 1(a) of this document.

A ketimine or polyketimine with a functional group bearing one active hydrogen atom is then added to an isocyanate functional prepolymer; in the subsequent addition reaction, an active hydrogen combines with an isocyanate group, yielding a ketimine (or polyketimine) terminated polyurethane oligomer, such as those of the formula ##STR3## wherein p represents a number ranging from 1.01-5, preferably 1.1-3, and, wherein R₃ represents the residue remaining of an isocyanate functional prepolymer after removal of the free isocyanate function. Such prepolymers can be prepared, as is well known in the art, by combining an excess of one or more monomeric polyisocyanates having at least two free isocyanate groups, with monomeric or polymeric compounds containing at least two active hydrogen atoms which are reactive with free isocyanate groups.

Representative monomeric polyisocyanates, while not being limited to these compounds, include; toluene diisocyanate, 4,4'-diphenyl diisocyanate, 4,4'-diphenyl-methane diisocyanate, 4,4'diphenyl-ether diisocyanate, dianisidine diisocyanate, 1,5-naphthalene diisocyanate, p-phenylene diisocyanate, trimethylene diisocyanate, octadecylmethylene diisocyanate, 2'-chloropropane diisocyanate, 4,4'-methylene-bis(phenyl isocyanate), isophorone diisocyanate, 1,6-hexamethylene diisocyanate, 4,4'-methylene-bis(cyclohexyl isocyanate), 4,4'4"-triphenyl-methane triisocyanate, 1,3,5-benzene triisocyanate, poly methylene poly(phenyl isocyanate), and/or di or tri-isocyanate functional adducts of the above mentioned monomers with polyhydroxy monomers, such as the adduct of one mol of trimethylolpropane with 3 mols toluene diisocyanate, or 3 mols of 4,4'-methylene-bis(cyclohexyl isocyanate).

The literature is replete with listings of suitable monomeric or polymeric compounds containing at least two active hydrogen atoms which are reactive with free isocyanate groups, including; water, mercaptans, primary and secondary amines, acids, and hydroxyl containing compounds. Hydroxyl containing compounds are currently the materials of choice for the preparation of isocyanate functional prepolymers for the purposes of this invention. Representative hydroxyl containing compounds include; ethylene glycol, diethylene glycol, 1,3-propylene glycol, 1,4-butane diol, glycerol, trimethylolpropane, erythritol, pentaerythritol, polyethers, such as poly(ethylene oxide) diol, poly(ethylene oxide/propylene oxide) diol, poly(propylene oxide) diol, and poly(tetramethylene oxide) diol, polyactones, such as polycaprolactone, and polyhydroxypolyesters of polycarboxylic acids, such as succinic acid, adipic acid, suberic acid, azelaic acid, sebacic acid, phthalic acid, isophthalic acid, and teraphthalic acid with polyols such as ethylene glycol, diethylene glycol, 1,4-butane diol, trimethylolpropane, glycerol, erythritol, pentaerythritol, poly(ethylene oxide) diol, poly(ethylene oxide/propylene oxide) diol, and poly(tetramethylene oxide) diol.

Preparation of the isocyanate functional polyurethane prepolymer can be by the conventional one-stage process, whereby the reactants, including at least one polyisocyanate having at least two isocyanate groups, at least one organic compound having at least two active hydrogen functional sites, and preferably having at least two hydroxyls as the source of the active hydrogen atoms, and which preferably has a molecular weight in the range of 200 to 7500, and optionally, at least one chain-extending organic compound having two active hydrogen atoms, or chain-branching organic compound having at least three active hydrogen atoms, and which preferably has a molecular weight below 300, are mixed together and reacted simultaneously, employing an isocyanate:active hydrogen ratio of from 1.01:1 to 5:1, preferably 1.1:1 to 3:1.

Preparation of the isocyanate functional polyurethane prepolymer may be carried out in the melt or in solution, that is, in the absence or presence of organic solvents which are inert with respect to isocyanate moities. Perferred organic solvents for preparation of this isocyanate functional prepolymer in solution are those that are substantially soluble in water, or miscible with water, since the dispersion of the subsequently formed ketimine terminated urethane oligomer into water is greatly facilitated by the coupling action of the organic solvent with water to solubilize and disperse the resin.

Organic solvents found suitable for this preparation include, but are not limited to; methyl acetate, ethyl acetate, amyl acetate, acetone, methylethyl ketone, diethyl ketone, methylisobutyl ketone, dimethyl formamide, dioxane, and methyl pyrrolidone. Prepolymer preparation should be carried out in anhydrous conditions at a temperature in the 50°-80° C. range for several hours.

It is well known to those skilled in the art that the physical and performance properties of coatings cast from solvent solutions of polyurethane prepolymers can be adjusted almost infinitely by varying certain prepolymer parameters. It has been found that the properties of aqueous urethane coatings are likewise influenced by prepolymer design; that is to say the design of the primary amine functional urethane oligomer primarily determines whether the cured coating will exhibit properties that compare favorably with existing solvent borne one component coatings.

For the most part, the well-known, well-understood design principles for urethane prepolymer solution coatings are applicable in designing these aqueous urethane polymer coatings, with one important exception, which relates to the size of the prepolymer molecule. In urethane prepolymer solutions it is necessary to use a substantial excess of polyfunctional isocyanate monomer in order to achieve suitable shelf stability. Even when prepolymers are essentially linear from combining only difunctional reactants, the combining ratio of isocyanate:hydroxyl equivalents must exceed about 1.6, resulting in prepolymers of high isocyanate functionality, and low equivalent weight per unreacted isocyanate functional group that typically falls into the range of 350-1000. These prepolymers cure upon application through isocyanate:isocyanate or isocyanate:hydroxyl interactions, both being essentially chain-extending types at ambient curing conditions. Thus, coatings made from polyurethane prepolymer solutions develop their characteristic properties as the result of chain extending reactions that occur during cure.

The prepolymer to be used in this waterborne system, on the other hand, cures through lacquer dry, that is the size of the prepolymer is large enough and crosslinked and/or chain extended to provide sufficient performance characteristics, simply upon evaporation of the water, acid and any cosolvent, if used. That is these polymers are solid resins. This prepolymer must be sufficiently linear in design and possess sufficient chain-branching segments, and be of a sufficient molecular size, prior to determination of isocyanate functionality, if it is to possess "urethane-like" characteristics in the dry film state. To accomplish this, isocyanate:hydroxyl combining ratios below 2.0 are employed; and these may range from about 1.01 to about 2.0, preferably between about 1.1 to about 1.5. The isocyanate equivalent weight of the prepolymer could range as low as about 250 to about 5000, but preferably should range from about 750 to about 4000, to accomplish the purposes of this invention.

The addition reaction resulting from combining a mono or polyketimine with a functional group bearing an active hydrogen atom and an isocyanate functional prepolymer should also be carried out in anhydrous conditions, since ketimines are readily hydrolyzed, and free isocyanates are reactive with water. Reaction conditions vary according to the nature of the functional group bearing the active hydrogen atom on the ketimine. When the functional group is a secondary amine, the addition reaction with an isocyanate links the ketimine and the urethane prepolymer through a substituted udrea linkage, a reaction that occurs very rapidly, even at ambient temperature conditions. The reactants are combined using vigorous agitation. The reaction is accompanied by a substantial increase in viscosity. When the functional group bearing an active hydrogen atom is a hydroxyl, reaction time and temperature should be increased since the rate of the isocyanate:hydroxyl reaction, linking the ketimine molecule to the prepolymer through a urethane linkage, is much slower than the isocyanate:secondary amine rate. The isocyanate:thio reaction, linking a monothio, nono or polyketimine to an isocyanate functional prepolymer through a thiourethane linkage, occurs at a rate somewhat slower than the isocyanate:hydroxyl rate and the reaction time, in particular, should be prolonged, and the reaction temperature should be increased to the 90°-100° C. range.

Once the urethane-ketimine oligomer is formed, there usually is no reason for maintaining the product in anhydrous conditions. Hydrolysis of the ketimine can occur with exposure of the product to moisture, but the product is stable in either condition.

The ketimine terminated oligomer can be dispersed in water by neutralizing a portion of the amine functionality with a volatile solubilizing acid. In the presence of a salt, or acid neutralized resin, the oligomer spontaneously disperses in water. Simultaneously, hydrolysis of the imine structure occurs; the ketone is regenerated, and the primary amine functionality is restored, as such, or as a primary amine salt, since a portion of the amino nitrogens have been neutralized with the volatile solubilizing acid, as indicated schematically by the formulae: ##STR4##

The use of volatile solubilizing acids to disperse amine functional polymers is well-known art, and any of the well-known acids may be used to neutralize the oligomer. Acetic acid is commonly used for this purpose, and is preferred when the end product's intended use is that of a room temperature-curing protective coating system. It is beneficial to select an acid that volatilizes readily from a thin liquid film of the applied product in ambient conditions.

It has been found that the properties of the ultimate urethane polymer coating system are influenced by the quantity of the solubilizing acid used to neutralize the urethane oligomer, or the percentage of amine functionality that is neutralized. It has been found, that the ultimate ambient-cure properties (e.g. 2 weeks cure time at 75° F., 50% relative humidity) of coatings utilizing completely neutralized amine functional urethane oligomers, and even systems with 50% neutralization, have important performance property deficiencies, when compared to similar systems employing oligomers with somewhat less than about 50% of the amine function neutralized.

It might also be expected, however, that amine functional urethane oligomer dispersions with somewhat less than complete neutralization of amine functionality, and especially with somewhat less than 50% neutralization would exhibit reduced shelf stability. Indeed, it has been found that when neutralization is reduced to less than about 20%, the resulting dispersion has a relatively high viscosity, is often lumpy, and irreversible coagulation of the dispersion occurs within only a few days. Dispersions of oligomers neutralized from about 25% of stoichiometry to complete neutralization, however, exhibit essentially identical dispersed state characteristics of viscosity, translucency, and shelf life.

A variety of neutralizing and dispersing techniques can be employed to disperse the oligomer;

Concentrated acetic acid may be added directly to the oligomer solution (resulting in a significant resin viscosity increase). The viscous resin can then be slowly added to water, with vigorous agitation, or water may be slowly added to the resin, with vigorous agitation.

The oligomer solution may be neutralized with dilute acetic acid (e.g. 10%-50% acid by weight). The viscosity of the solution increases, but not as severely as when concentrated acid is used. The acidified resin can then be added to water or water can be added to the resin.

The calculated weight of acid can be added to the water of dispersion and the unneutralized resin can be added to acidified water, or the acidified water can be slowly added into the unneutralized resin.

Vigorous agitation is essential for each of the dispersion options, although somewhat less than high-shear agitation is needed. A somewhat viscous colloidal dispersion of solvent swollen resin particles in water results. The acid neutralized oligomer spontaneously disperses without the aid of a surfactant or emulsifying agent.

The dispersion at this point normally contains the organic solvents used to prepare the isocyanate functional prepolymer, plus a quantity of regenerated ketone from the hydrolysis of imine that occurs spontaneously when the ketimine terminated oligomer is dispersed in water. This dispersion can be used as is to provide a urethane polymer protective coating system with useable ambient-cure properties. It can readily be appreciated, however, that the above amine terminated oligomeric dispersion contains undesired high levels of volatile organic solvent, and therefore, the above described coating system could pose serious toxicity and/or flammability concerns during application. In the interest of providing a coating system that minimizes these concerns, and one that meets current E.P.A. requirements for V.O.C., it is preferred that the volatile materials are removed. The choice of solvents used for prepolymer processing, and the choice of the ketone for the synthesis of the ketimine have a major impact on the success of the solvent stripping operation.

Organic solvents, including the ketones, with boiling ranges considerably higher than that of water may be difficult to strip. Another benefit from the removal of the organic processing solvents and the regenerated ketones is that the viscosity of the dispersion reduces as solvents are removed, apparently from a reduction in resin particle size as solvent is extracted.

It is well known to those skilled in the art that glycol ethers or blends of glycol ethers and other organic solvents in waterborne coating systems can provide a number of useful functions in addition to functioning as a coalescing aid. They can improve the flow properties of the coating, can help control sagging and running, and can influence the evaporation of the water/solvent/volatile solubilizing acid system. A variety of ethers of ethylene, diethylene, and propylene glycols are utilized commercially in waterborne coating systems; the butyl ether of ethylene glycol (e.g. Butyl Cellosolve, Union Carbide; Ektasolve EE, Eastman Chemical Products, Inc.), is a widely used example of this type solvent, often called a cosolvent.

The quantity of cosolvent addition may range from 0 to about 20% by weight of the total applied coating, with a range of less than 1.5% preferred. In these quantities atmospheric vapor concentration levels of these low volatility type solvents during application in ambient conditions should not constitute an appreciable safety and/or health hazard.

In the present invention it has been found that the above differences in polymer configurations can be incorporated with advantage. It has been pointed out previously that urethane-type properties of these coatings are built into the original urethane prepolymer, and that larger prepolymer molecules with slight to substantial branching are useable, depending on the properties sought.

An evaluation of experimental aqueous urethane polymer coatings was conducted to determine if such coatings could meet the objectives of providing properties similar to those of existing solvent borne coatings. The considerations involved in test method selection, and a description of methods chosen is as follows.

Perhaps the predominant performance feature of polyurethane coatings which sets them apart is their "toughness". The Paint/Coatings Dictionary, published by the Federation of Societies for Coatings Technology, 1978, defines toughness as "that property of a material by virtue of which it can absorb work." Brittle is defined in the same volume as " . . . the opposite of tough." Possibly a measure of the resiliency of a protective coating film would be a good measure of its toughness; unfortunately, this parameter is difficult to measure in films whose thickness ranges from5-80 microns (0.2-3.15 mils). Abrasion resistance is a commonly measured property in the protective coatings industry and is closely related to toughness, as it measures the work absorbing capacity of a coating.

A variety of abrasion resistance test techniques for protective coatings are published by ASTM. Method D-4060-81, "Abrasion Resistance of Organic Coatings by the Taber Abraser," is perhaps the most popular. Method D-658-81, "Abrasion Resistance of Organic Coatings by the Air Blast Abrasives Test", is believed by many to provide more consistent values from run to run, or season to season, and has been the method used in measuring the abrasion resistance of the coatings of this invention. Abrasion resistance values by this test are reported in terms of grams of silicon carbide grain abrasive required to remove 1 mil of coating under specified test conditios; in general, the higher the value, the greater the ability of the protective coating to resist abrasion. This definition has to be qualified, however, because formulations that are very soft and extensible, but yet resilient, provide the highest abrasion resistance values, but are often too soft to be useful as protective coatings.

Typical clear/pigmented protective coatings yield abrasion resistance values according to the air blast abrasive test as follows:

Alkyds: 35-70 (g/mil)

Epoxy esters: 40-80

2 part solution epoxies: 55-90

Oil-modified urethanes: 40-85

Film hardness in this study has been evaluated with 2 measuring techniques, including the well-known Sward hardness rocker, which measures the damping efect of a coating on the rocking action of a two blade rocker, previously calibrated on plate glass. Calibration results are adjusted to yield a value of 100 on plate glass so that Sward hardness of a coating can be interpreted as a percentage of the value on plate glass. Generally speaking, Sward hardness values for commercially available protective coatings, with rare exception, fall in the range of about 15 (very soft) to 70 (very hard). One drawback of the method is that surface imperfections such as dust in the film, trapped bubbles, or poor leveling tends to further dampen the rocking action, thereby yielding values somewhat less than actual.

ASTM method D-4366-84, "Hardness of Organic Coatings by Pendulum Damping Tests" describes the Konig hardness tester, the other hardness measurement used in this evaluation. This method is preferred by many over the Sward test, since coating surface imperfections have little influence on the hardness reading. Konig hardness is defined (ASTM) as "time in seconds for the swing amplitude of the Konig pendulum to decrease from 6° to 3°". This unit is also calibrated on plate glass, yielding a value of 250±10 seconds. On harder coatings the Konig value approximates 2.5 times the Sward value; on softer coatings the difference is slightly less.

The water resistance of the experimental and control coatings of this study was evaluated by depositing puddles of distilled water on the surface of coatings applied and cured on glass in standard conditions of 73° F., 50±5% relative humidity for two weeks. The effect of exposure to water was evaluated immediately after removal of the water, and again after a 1 hour recovery period. Evaluations were made after water exposures of 1 hour, 4 hours, and 24 hours duration. In all cases, evaluations included the performance of a control coating, a commercially available oil modified urethane varnish. Water resistance performance ratings of the experimental coatings were based on comparisons with the control.

The following examples set forth the sequential steps for preparing the coatings, and the results obtained from the above testing regimen.

EXAMPLE 1 FORMATION OF THE KETIMINE

The condensation product obtained by reacting N-(aminoethyl) piperazine and methylethyl ketone results in release of water and formation of a ketimine. The primary amine is thus "blocked" from reacting with isocyanates. Suitable amines may be mono (primary) functional or multifunctional. Any non-cyclic ketone can be used to form the ketimine, but methylethyl ketone (MEK), and methylisobutyl ketone (MIBK) are preferred due to practical considerations.

EXAMPLE 1 (a)

In a typical preparation, a gram mol of N-(aminoethyl) piperazine (AEP, 129 g), is combined with 1.1 gram mol of MEK (79.2 g) in a nitrogen blanketed reaction flask along with approximately 200 g of cyclohexane. The mixture is heated to reflux temperature (80°-85° C.) and is refluxed until the stoichiometric quantity (18 g) of water has been collected in a Dean-Stark trap. Cyclohexane and excess MEK are then removed from the ketimine in a rotary evaporator. Infrared scans of the ketimine are made to insure the development of absorption due to C=N formation (1660 cm⁻¹), and the disappearance of absorption due to C=O at 1710 cm⁻¹, and the disappearance of absorptio due to NH₂ at around 1600 cm⁻¹. The amine equivalent weight of the product is determined to be 195.

EXAMPLE 1 (b)

A gram mol of diethylene triamine (DETA, 103 g) is combined with 2.2 mol MIBK (220 g), and approximately 300 g cyclohexane, using the procedure of Example 1 (a). After removal of the stoichiometric condensed water (36 g), the reflux solvent and excess MIBK are removed. The equivalent weight of the product is determined to be 273.

EXAMPLE 2 Preparation of the Urethane Prepolymer

The base isocyanate prepolymer can be prepared from a variety of polyols and isocyanate prepolymers. The possibilities are endless and are familiar to those skilled in the art. The following is typical:

EXAMPLE 2 (a)

375 g of polypropylene glycol, hydroxyl number (OH#)-149.6 (1.0 equivalents of hydroxyl), 44.66 g of trimethylolpropane (TMP, 1.0 equivalents of hydroxyl), and 217.5 g of toluene diisocyanate (TDI, 2.5 equivalents of isocyanate) are combined in a two liter glass reaction flask under a nitrogen atmosphere. A reaction temperature of 65+2° C. is maintained by incremental addition of 424.8 g of dry MEK (<0.05% water). After 8 hours the isocyanate content is 1.96% (theoretical, 1.98%). Non volatile content (N.V.) is 60.1%, Brookfield viscosity-265 centipoise. The free isocyanate (NCO) equivalent weight of the prepolymer, solids basis, is 1288, and is 2142.8, solution basis. It is listed as Prepolymer 1 in Example 2 (b).

EXAMPLE 2 (b)

The following prepolymers were prepared by the procedure described in Example 2 (a).

    ______________________________________                                         Pre-                                                                           polymer                                                                               Type of  Type of           %     equiv.                                 ID     Glycol   Diisocyanate                                                                              NCO:OH NCO   wt. solids                             ______________________________________                                         1      PPG      TDI        1.25   1.96  1288                                   2      PPG      TDI        1.4    2.27  931                                    3      PPG      TDI        1.68   4.06  533                                    4      PPG      TDI        2.0    4.98  411                                    5      *PPG     TDI        1.4    2.96  695                                    6      PPG      TDI        1.6    3.23  788                                    7      PTMEG    TDI        1.4    2.79  858                                    8      PTMEG    TDI        1.67   5.25  584                                    9      Cap      Des W      1.5    2.63  958                                    10     PPG      MDI        1.67   2.51  1004                                   11     PTMEG    MDI        2.0    2.3   1196                                   12     PPG      IPDI       1.5    2.95  859                                    13     PE       Des W      1.67   1.90  1121                                   ______________________________________                                    

PPG-Polypropylene glycol, OH#-149.6

*PPG-Polypropylene glycol, OH#-112.4

PTMEG-Polytetramethylene glycol, OH#-112.1

Cap-Polycaprolactone, diol, OH#-214

PE-Poly (Neopentyl Adipate) OH#-111.4

Des W-Bis (4-isocyanatocyclohexyl)hexane

IPDI-Isophorone diisocyanate

TDI-Toluene diisocyanate

MDI-4,4-methylene-bis(phenyl isocyanate)

EXAMPLE 3 Formulation of the Ketimine Blocked urethane Oligomer

This is a straight stoichiometric addition reaction. The ketimine is reacted under anhydrous conditions at the secondary amine site with a urethane prepolymer having a reactive terminal isocyanate group resulting in a urethane oligomer terminated by the ketimine. Thus the functional isocyanate group reacts with the secondary amine as it is prevented from reacting with the primary amine due to the ketimine mask.

EXAMPLE 3 (a)

96.6 g of Example 1 (a) is combined with Example 2 (a) in a 2 liter glass reaction flask under a nitrogen atmosphere with vigorous agitation at a temperature of 40° C. After 20 minutes, an infrared scan indicated a complete reaction by the disappearance of absorption due to unreacted NCO at 2250 cm⁻¹.

EXAMPLE 3 (b)

The 13 prepolymers of Example 2 (b) were combined with the monoketimine of Example 1 (a) or the polyketimineof Example 1 (b) or were divided so portions could be combined with each, by the procedure of Example 3 (a) as follows:

    ______________________________________                                         Prepolymer ID                                                                              Oligomer ID                                                                               Terminal Amine Group                                    ______________________________________                                         1           1 (a)      AEP                                                     1           1 (b)      DETA                                                    2           2 (a)      AEP                                                     2           2 (b)      DETA                                                    3           3 (b)      DETA                                                    4           4 (a)      AEP                                                     5           5 (a)      AEP                                                     5           5 (b)      DETA                                                    6           6 (b)      DETA                                                    7           7 (a)      AEP                                                     7           7 (b)      DETA                                                    8           8 (b)      DETA                                                    9           9 (a)      AEP                                                     10          10 (a)     AEP                                                     11          11 (a)     AEP                                                     12          12 (a)     AEP                                                     13          13 (a)     AEP                                                     ______________________________________                                    

EXAMPLE 4 Neutralizing and hydrolyzing the Blocked Oligomer to Form an Amine-Terminated Urethane

The ketimine terminated oligomer is neutralized with a volatile organic acid to render it water dispersible and hydrolyzable. Water is then introduced to hydrolyze the ketimine, restoring the primary amine functionality and regenerating the ketone, which is subsequently stripped, along with other processing organic solvents.

The following product of neutralization and hydrolysis is presented with the understanding that the ammonium acetate (cationic terminal group) is approximately 33%, the remainder being terminated by NH₂.

EXAMPLE 4 (a)

Glacial acetic acid is added to the product of Example 3a with vigorous agitation, resulting in a significant solution viscosity increase. 234 g. MEK is added to reduce viscosity and after thorough blending, the acidified product is slowly added to 1995 g. distilled water, using vigorous low shear agitation. A colloidal dispersion of the solvent-swollen resin in water results, (N.V.-20.7%, Brookfield viscosity-165 cps). As MEK is stripped from the dispersion in a laboratory rotary evaporator, the viscosity gradually drops. When 680 g. MEK, plus approximately 5 g. water has been collected, and when the distillate appears to be composed of water entirely, the stripping is discontinued. The final product characteristics are:

N.V.%: 25.8

Viscosity: 8 cps (Brookfield)

pH: 7.05

Appearance: Slightly amber translucent

Organic solvent content-0.85% MEK via gas chromatographic (G.C.) analysis

EXAMPLE 4 (b)

The 17 oligomers of Example 3 (b) were dispersed in water following the procedure of Example 4 (a), as follows:

    ______________________________________                                                 %        %                Active Hydrogen                              Oligomer                                                                               Neutrali-                                                                               N.V.     Viscosity                                                                              Equivalent                                   ID      zation   Solids   cps     wt./solution                                 ______________________________________                                         1 (a)   33       25.8     8       2517                                         1 (b)   33       25.8     10      1231                                         2 (a)   33       30.2     240     1755                                         2 (b)   33       33.2     500     780                                          3 (b)   25       31.4     100     560                                          4 (a)   33       38.2     87      780                                          5 (a)   33       31.3     200     1319                                         5 (b)   33       34.1     420     571                                          6 (b)   33       27.3     21      816                                          7 (a)   33       29.5     40      1674                                         7 (b)   33       28.0     55      924                                          8 (b)   33       35.1     65      482                                          9 (a)   33       32.5     32      1673                                         10 (a)  33       34.0     66      1667                                         11 (a)  33       31.8     101     2084                                         12 (a)  33       30.3     380     1630                                         13 (a)  33       34.0     88      1839                                         ______________________________________                                    

In summary, the following statements are appropriate to the entire disclosure. Monosecondary (NH) mono or difunctional primary (NH₂) amines are preferred and specified herein. However, an amine may be used in which the monosecondary amine function is replaced by another functional group presenting a single active hydrogen. A ketimine with hydroxyl functionality is possible, for example.

We have referred to the limited range of neutralization as about 27%-37%. This is preferred as the safe range but it may be mentioned that no substantial drop-off in properties is observed until neutralization is in the range of 40-43%; likewise, the lower limit is not well defined and may reach to 20% or even less in some instances.

Hence, while we have illustrated and described the preferred limits and preferred embodiments it is to be understood these are capable of variation and modification for actual practice within the purview of the appended claims. 

What is claimed is:
 1. A method of preparing a primary amine terminated urethane oligomer in aqueous dispersion form to result in an ambient curing (room temperature curing, e.g. 70° F.) urethane polymer coating comprising the steps of:A. producing a mono or polyketimine by reacting a monoketone and the primary amine functionality of a mono or polyfunctional primary amine compound that contains one other functional group bearing one active hydrogen atom that is reactive with isocyanate functionality, and; B. producing an isocyanate terminated prepolymer of an equivalent weight per unreacted isocyanate group ranging from about 250 to about 5000, and preferably from about 750 to about 4000 by combining suitable polyfunctional hydroxyl bearing monomers and/or polymers with an excess of suitable aromatc and/or aliphatic polyfunctional isocyanate monomers, preferably difunctional isocyanate monomers, and subsequently terminating the prepolymer reactive isocyanate functionality by an essentially stoichiometric addition of the mono or polyketimine, based on the free hydrogen atom equivalency of the mono or polyketimine compound, whereby the addition reaction yields a mono or polyketimine terminated urethane oligomer substantially free of residual isocyanate functionality, and; C. neutralizing to salt form part of the amine nitrogens of the oligomer with a volatile acid and dispersing in water both the neutralized and unneutralized oligomer, thereby hydrolyzing the ketimine to restore the primary amine functionality and to regnerate the ketone which is subsequently stripped, yielding an aqueous oligomer dispersion with only residual volatile organic content.
 2. A method according to claim 1 in which the ketone of step A is methylethyl ketone or methylisobutyl ketone in which the amine is N-(aminoethyl) piperazine or diethylene triamine and in which the range of neutralization in step C is between 27-37 percent.
 3. A method according to claim 1 in which steps A and B are stoichiometric, in which the amount of neutralization according to step C is in the range of 27-37 percent.
 4. An aqueous system to be spread on a substrate as a coating, substantially devoid of volatile hazardous organic solvents comprising an aqueous dispersion of a urethane oligomer terminated by primary amine groups, and in which a portion of the amine groups is transformed to a salt by neutralization with an acid.
 5. A system according to claim 4 in which the amine salt equivalent is in the range of 27-37 percent of the amine function present.
 6. A system according to claim 5 in which the urethane oligomer is one in which N-(aminoethyl) piperazine is coupled at its secondary amine site to the isocyanate functinal group of a urethane prepolymer prepared by reacting a polyglycol with excess organic diisocyanate.
 7. An aqueous system to be spread on a substrate as a coating, substantially devoid of volatile hazardous organic solvents comprising an aqueous dispersion of a urethane oligomer terminated with primary amine functional groups, such oligomer being the reaction product of an isocyanate terminated prepolymer and a mono or polyketime compound containing one other functional group bearing a single active hydrogen atom, the site of the terminating addition, such isocyanate terminated prepolymer resulting from combining suitable polyfunctinal hydroxyl bearing monomers and/or polymers with an excess of suitable aromatic and/or aliphatic polyfunctional isocyanate monomers, preferably difunctional isocyanate monomers, such reactants being selected and combined in such proportions to yield essentially linear or moderately branched chain prepolymer molecules of an average equivalent weight per unreacted isocyanate function ranging from about 250 to about 5000, and preferably from about 750 to about 4000, such ketimine terminated oligomer being dispersed into water aided by neutralization of 25-40% of the amine function to an amine salt by a volatile organic acid, such dispersion subsequently being stripped of volatile processing solvents as well as the regenerated ketone formed by hydrolysis of the ketimine or polyketimine during dispersion.
 8. A system according to claim 7 whereby the oligomer is terminated by either N-(aminoethyl)piperazine or diethylene triamine, herein the ketimine/polyketimine block is formed by reaction of the primary amine active hydrogen atoms and the carbonyl of methylethyl ketone or methyl isobutyl ketone.
 9. A system according to claim 8 in which the amine salt equivalent is in the range of 27-37 percent of the amine function present.
 10. A system according to claim 7 in which the urethane oligomer is one in which N-(aminoethyl) piperazine or diethylene triamine is coupled at its secondary amine site to the isocyanate functional group of a urethane prepolymer prepared by reacting a polyglycol with excess organic diisocyanate.
 11. A method of preparing a primary amine terminated urethane in aqueous dispersion form comprising the steps of:A. producing a ketimine by reacting a ketone and an amine having both primary amine functionality and one other functional group containing a single active hydrogen; B. reacting the ketimine with a urethane prepolymer terminated with a reactive isocyanate group, resulting in a ketimine terminated urethane oligomer whereby the addition reaction renders the oligomer substantially free of any residual isocyanate functionality; C. neutralizing to salt form part of the amine nitrogens of the oligomer with a volatile acid and dispersing in water both the neutralized and unneutralized oligomer, thereby hydrolyzing the ketimine to restore the primary amine functionality and to regenerate the ketone which is subsequently stripped, yielding an aqueous oligomer dispersion with only residual volatile organic content.
 12. A method according to claim 11 in which the ketone of step A is a methylethyl ketone, in which the amine is N-(aminoethyl) piperazine and in which the range of neutralization in step C is between 27-37 percent.
 13. A method according to claim 11 in which steps A and B are stoichiometric, in which the amount of neutralization according the step C is in the range of 27-37 percent. 